Great advice - hope to stumble upon the fountain rather than searching for it. This is how most people do it, O think, and it makes for more interesting sight-seeing. The same goes for Piazza di Spagna and the Spanish Steps.
Navigating the area around the fountain can be confusing...finding it, as if you were lost and unexpectedly came upon it is most satisfying... Maybe explore the area rather than explicitly planning on seeing it?
I love Rome and Trevi fountain is one of the reasons why. You turn a corner and see something new and amazing. One moment you are walking down a narrow street the next you are looking at an amazing piazza.